An issue has come to light with Microsoft Windows 2016 Standard Edition - if Windows updates have not been run before Active Directory services are installed you will see incorrect information from the selection list in regards to the forest and domain function level.

This is a known bug with Windows Server 2016 and running the latest Windows updates will rectify the issue. It does not affect the functionality of Active Directory in any way - it is quite simply a label that wasn't updated in the RTM.